Magnetron overload protection circuit



Jan. 21, 1958 R. K-F SCAL 2,320,900

MAGNETRON OVERLOAD PROTECTION CIRCUIT Filed July 30, 1954 CATHODE -QWEBWY MODULATOR MAGNETRON INVENTOR ROBERT K-F SGAL" ATTORNEYS United States Patent MAGNETRON OVERLOAD PROTECTION CIRCUIT Robert K-F Scal, Englewood, N. J., assignor to the United States of America as represented by the Secretary of the Navy Application July 30, 1954, Serial No. 446,983

5 Claims. (Cl. 25036) (Granted under Title 35, U. S. Code (1952), see. 266) Because of the danger of damage to the magnetron or other parts of the system when the magnetron misfircs, radar sets requires some type of protection device. A

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2,820,900 Patented Jan. 21, 1958 magnetron modulator of a type will known in the art and described, for example, in chapter VI of Principles of Radar, second edition, M. I. T. Radar School, McGraw- Hill Book Company, New York, 1946, or in chapter 111 of Principles of Radar, third edition, M. 1. T. Radar School Staff, McGraw-Hill Book Company, New York, 1952.

- A capacitor is connected in parallel with resistor 13 common type of device utilizes a relay controlled by a small bias voltage developed across a resistor placed in the high voltage D. C. power supply line ahead of the magnetron modulator. When the magnetron misfires or some other overload occurs, the bias voltage is increased, thereby actuatirglhe relay which opens the D. C. power supply to the magnetron "Emma? The magnetron e then ceases to function, thereby reducing the bias voltage to zero and allowing the relay to close again. If the overload has not been removed, the circle is repeated and the relay chatters on and off. The operation of devices of this type is unpredictable because of the complexity and relatively low degree of reliability of mechanical relays. The overload protection circuit of the present invention avoids the use of mechanical relays, thereby overcoming the disadvantages of the earlier known protection devices.

An object of the present invention is the provision of a simple but reliable magnetron overload protection circuit.

Another object is the provision of a magnetron overload protection circuit which does not rely upon interruption of the power supply to the magnetron modulator when an overload or short circuit occurs.

An additional object is to provide a magnetron overload protection circuit which reduces the repetition rate of the trigger generating means for the magnetron modu lator when an overload occurs.

A further object is to provide a magnetron overload protection circuit which cuts out the trigger generating means for the magnetron modulator when an overload occurs.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawing wherein:

The figure shows a circuit diagram, partly in schematic, of the present invention.

Referring more particularly to the drawing, shown schematically at 11 is a high voltage D. C. power supply having its positive and negative terminals connected to ground and to a magnetron modulator 12, respectively. One end of a resistor 13 is connected to the positive terminal ground connection and the other end is joined to the magnetron modulator by lead 14. Magnetron modulator 12 may be a conventional trigger pulse controlled providing a oy-pass for high frequency pulse signals. A trigger generating means, such as a master blocking oscillator consisting of triode 16, blocking oscillator transformer 17, capacitor 18 and variable resistor 19 is con nected through the cathode follower 21 of the magnetron modulator, which is in turn coupled to the magnetron 22. The magnetron modulator develops a D. C. voltage across the resistor 13 proportional to the high voltage supply current under normal conditions. This voltage is utilized to bias the master blocking oscillator and is applied to the grid of triode 16 through lead 23. When an overload occurs in the circuit illustrated, the high voltage supply current increases and the bias voltage increases proportionally. The increased bias voltage applied to the grid of triode 16 reduces the repetition rate of the master blocking oscillator, thereby reducing the overload current. At a particular repetition rate, an equilibrium will attain where the abnormally large current in resistor 13 will maintain a low repetition rate in the master blocking oscillator until the fault has cleared and the operation returns to normal. If the overload current is sufliciently high, the master blocking oscillator will be cut off entirely, thereby stopping operation of the magnetron modulator. This, of course, cuts off the current in the resistor 13, so that the blocking oscillator quickly resumes operation.-- lfctheianltncontinues, the blocking oscillator is again out off for a short period. In this way, by repeated cycling, the current is held to a safe value until such time as the fault clears and normal operation is automatically resumed.

Ordinarily, the master blocking oscillator is adjusted, by means of variable resistor 19, so that its repetition rate remains constant when the high voltage supply current is one to one-and-one-half times its normal value. When the current increases further, the repetition rate decreases due to the increased bias, and when the current reaches about twice its normal value, the blocking oscillator ceases to operate completely. However, when other factors make it necessary to keep the repetition rate constant, a gaseous discharge tube, such as the neon bulb 24, is placed between the overload bias resistor 13 and the grid of triode 16. In such case, the blocking oscillator may be operated without external bias, and small normal current variations through resistor 13 are isolated from the grid of triode 16 by tube 24 and have no effect upon the repetition rate of the blocking oscillator. However, any

sufficiently large overload current developed across resister 13 causes tube 24 to fire, thereby biasing triode 16 to cut off, thus stopping operation of the blocking oscillator. This in turn eliminates the overload current in resistor 13 causing tube 24 to extinguish and allowing normal operation to be resumed. So long as abnormal conditions exist in the magnetron or the magnetron modulator, the cycle will be repeated, thereby limiting overload current to a safe value.

The cathode follower 21 is merely an isolating device, such as a common cathode follower circuit, to prevent feedback between the magnetron modulator and the master blocking oscillator.

The magnetron modulator 12 can be any of several well known devices, such as a thyratron type line modulator or a hard tube modulator, for producing high voltage pulses to be applied to the magnetron.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a radar set, a modulator coupled to a magnetron; a high voltage power supply connected to the modulator; oscillator means, including atriode, connectedto the modulator; means connected betweenthe power supply and the modulator for developinga bias voltage proportional to the high voltage power, supply current, said means being coupled to the grid of the triode; whereby upon occurrenceof anoverload in the set the bias voltage biases the grid .of the triode to reducethe repetition. rate ofthe oscillator.

2. The combination recitedinclaim 1, wherein said means comprises'a resistor and capacitor-connected in parallel.

.3. A protection c rc t o r amagnetron compris n wo terminal ;for c nn ction 1to.,-.a;,,h igh voltage power upp y, m g et n modulator connected .to said, mag netron, high-frequency by -passed impedance means, circuit means connectingsaid magnetron; modulator in series with said impedance means to said two terminals where by during operation a .voltageis developed across said impedance means whichis proportional to the magnetron load current,vbias.-.controlled trigger generating means connected to pulse .said modulator, and circuit means coupling the voltage. developed by .said irnpedance means to said trigger generating means tobias said trigger-generating means whereby upon a magnetron overload the fre- 4 quency of the pulsingof said modulator is diminished and for large overloads is stopped.

4. The protection circuit of claim 3 wherein: the circuit means for coupling the voltage developed by said impedance means to saidtrigger means includes means for isolating from the trigger generating means the voltage developedby 'normalv current variationscthrough said impedance means but for'passing voltages due to magnetron overload currents.

,5. A protection circuit for .amagnetroncomprising: input terminal means adapted to be connectedrto, ah igh voltage power supply, a resistor and a capacitor connected in parallel, a magnetroma magnetron'modulator connected to said magnetron, circuit,meansconnecting said magnetron modulator in series with said parallel connected resistor and capacitor tOazSfild input termi-nalmeans, a blocking oscillator circuit including a triode, circuit meansincludinga cathodefoll w rcircuitcoupfing the ou p ;of;sai.d abloekiogpscillator circuit to said-magnetron mod lator, and circuit mean including ,a gaseous discharge device connecting the .side of said resistor gadjacent said modulatorto the; grid ofsaid triode.

References Cited in the file of this patent UNITED --ST-ATES "PATENT S 2,458,574 ,Dow .Jan. 11, 1949 2,564,687 Guenther, Aug. 21, ,1951 2,619,632 ,Krumhansl ,et a1. Nov.. 25, 19.52 2,694,149 Gross Nov. .9, 1954 

